EdgeGateway basic class & tests

* largely needed for pending OrgVdcNetwork work, but useful separately
remove no-arg test case, since language handles
add test to ensure @id is set on initialize
remove bogus duplicate name case
add a test for looking-up-and-getting-id
add missing get_edge_gateway fog facade
pull id-grabbing logic out into seperate method
Merge pull request #86 from alphagov/edge_gateway

EdgeGateway class, as per new Vdc class